Optical transmission systems built around optical fibers have become widely used for broadband communication of digital data. Amplitude shift keying (ASK) is a modulation technique that is widely used for transforming the digital data into optical pulses carried by the optical fibers. Timeslots are defined for each bit of digital data to be transmitted. Optical pulses are generated during timeslots corresponding to “one” bits but not during timeslots corresponding to “zero” bits. In optical systems, ASK is often a non-return-to-zero (NRZ) transmission format. Hence, ASK is sometimes referred to as NRZ-ASK.
Differential phase shift keying (DPSK) is a competing modulation technique that is coming into use. DPSK's coding scheme assigns phase shifts to numbers. Timeslots are again defined, but unlike ASK, an optical pulse is generated during each timeslot. The optical pulse in each timeslot is phase-shifted from the optical pulse in the preceding timeslot as a function of the coding scheme to transmit the digital data from a transmitter to a receiver. DPSK, while more complicated to modulate and demodulate than ASK, has proven to offer a higher optical signal-to-noise ratio (OSNR), making it distinctly advantageous for long-haul optical transmission systems. DPSK can be an NRZ transmission format. Hence, DPSK is sometimes referred to as NRZ-DPSK.
Irrespective of whether the chosen format is ASK or DPSK, an optical carrier must still be modulated. An apparatus typically chosen to modulate an optical carrier is a Mach-Zehnder modulator (MZM). As those skilled in the pertinent art are familiar, an MZM has a pair of waveguide arms coupled at their ends. The optical carrier enters the MZM and is split between the waveguide arms. Electrodes associated with the waveguide arms receive a drive signal bearing the digital data that is to modulate the optical carrier. The electrodes cause the index of refraction of the waveguide arms to change such that, when the optical carriers recombine at the output end of the MZM, the resulting superposition yields the desired modulated optical pulsetrain.
MZMs are designed to receive a particular drive signal and switch with a particular switching response time. The switching response time defines the upper limit of the bandwidth of the optical pulsetrain they can produce. It is an absolutely paradigmatic to those skilled in the art that if a higher bandwidth optical pulsetrain is desired, an MZM that has been designed to be faster is required.
Unfortunately, the faster an MZM is designed to switch, the more power it consumes. Reducing power consumption in optical transmission systems is an important design goal, especially for long-haul systems. To compound matters, the MZM power consumption problem is bound to worsen as bandwidths of optical pulsetrains increase in the future.
Accordingly, what is needed in the art is an MZM that has a faster switching response time but that does not require concomitantly more power to switch.